As the countdown the Season 4 continues, so do the casting updates. While it's no secret that while covering the remainder of the events of A Storm of Swords​, the fourth season of Game of Thrones ​will also feature material from the concurrent novels ​A Feast for Crows and ​A Dance with Dragons. Joel Fry and Mark Gatiss have been cast as Hizdahr zo Loraq and Tycho Nestoris, respectively, but this latest announcement is very interesting indeed.

Struan Rodger has been cast in the role of the mysterious Three-eyed raven that has haunted Bran Stark's dreams since the first season. Those who have not read the books are no doubt confused by this, as the raven has already appeared in the show and doesn't appear to have the ability to speak or have any human qualities whatsoever. But that bird is not what it seems.

When we last saw Bran in Season 3, he was heading out Beyond the Wall with Meera and Jojen Reed, Hodor and his direwolf Summer. In the book, the party is lead through the Haunted Forest by a mysterious figure known as Coldhands (though it is not yet known if he will appear in the show). He leads them to a cave that is occupied by the last remaining Children of the Forest, long thought to be extinct. To his shock, Bran discovers that the three-eyed crow is not a bird at all, but a man (or what was once a man).

They are brought before a pale, skeletal man in rotted, black clothing in a weirwood throne of tangled roots. His skin is white, aside from a red blotch on his neck and cheek. He has fine, white hair long enough to reach the earthen floor. He is missing one eye, while the other is red. Weirwood roots surround the the man and grow through his body, including his leg and his empty eye socket. His voice is slow and dry, as if he had forgotten how to speak.

What is the mystery of the three-eyed raven?

Even after reading the book, some wonder who this figure is, and the ​Tales of Dunk and Egg novellas hold the answer. He is a very famous figure is Westerosi history and (in my opinion) the most fascinating character in the entire A Song of Ice and Fire mythos.

Brynden Rivers is one of the famous Great Bastards of Aegon IV Targaryen. He was born to Lady Melissa of House Blackwood. He was an albino and had a wine-red birthmark on his cheek, said to be shaped like a bird. This earned him the nickname "Bloodraven". He had a strong hatred for his half-brother Aegor "Bittersteel" Rivers, which Bittersteel returned. This was due to Bloodraven's mother replacing Bittersteel's mother (Lady Barba of House Bracken) as Aegon's favourite as well as their mutual desire for their half-sister Shiera Seastar. Bloodraven also wielded the Targaryen ancestral sword Dark Sister, a gift from his father.

On his deathbed, Aegon IV legitimized all of his bastards, and ensuing conflict almost tore Westeros apart. Daemon Blackfyre made his own claim to the throne and sought to overthrow his trueborn brother Daeron II Targaryen. Bittersteel joined him his revolt, while Bloodraven remained loyal to the king. The rebellion ended in what became known as the Battle of the Redgrass Field, where Bloodraven led the Targaryen loyalists to victory alongside Baelor Targaryen and Maekar Targaryen. He killed Daemon and his two eldest sons and lost an eye fighting against Bittersteel.

As a reward for helping to crush the Blackfyre Rebellion, Daeron named Bloodraven Hand of the King and Master of Whisperers, which inspired the riddle "How many eyes does Lord Bloodraven have? A thousand eyes, and one.". However, his extreme methods and kinslaying earned him the hatred of the realm, with many believing him to be a sorcerer.